Validator - validate that glTF batchId attributes are in range

[lilleyse]

@Rudraksha20

This will require parsing the gltf model. Each mesh in the gltf has a list of primitives, each of which contains vertex attributes. If the _BATCHID attribute is present, extract those values from the accessor/bufferView/buffer and then check that the values are all less than the BATCH_LENGTH property in the feature table.

[lilleyse]

For parsing, you can reference the code from here: https://github.com/AnalyticalGraphicsInc/gltf-pipeline/blob/cleanup/lib/parseGlb.js#L87-L118